A difficulty arises whenever instruments are utilized to measure anything. It has been said that no perfect measurement may ever be made for the reason that the very act of measuring the thing changes it. Needless to say, despite the fact that this is very often if not always true, in most circumstances that degree of precision is not warranted and any errors due to the act of measuring itself are well within acceptable limits for error. On the other hand, it is necessary for instruments to remain functioning at the highest possible levels for as long as possible for obvious reasons. Instruments which cease to function accurately within accepted limits after only short periods of use increase demands on the user's resources of time and money.
Methods for extending the operational time of instruments using sensors for taking measurements in various situations have long been sought. Many such sensors, including sensors for measuring temperature, dissolved oxygen, pH, gas constituencies, conductivity, turbidity, flow, color, biological activity, specific ionic activity, pressure, and oxidation-reduction potential, for example only, are designed to operate when immersed in fluids, including air and water. Often, such sensors are continuously and progressively degraded by the fluids within which they are immersed (perhaps by the very analytes for which the sensors are designed) such that the instruments cease to accurately measure that which they are calibrated to do. This is called “sensor drift” and might be caused by the nature of the sensor, its maintenance condition or by sensor fouling.
For the purposes of example only, and not by limitation, the purposes for such measurements and the need for such measuring instruments vary from environmental protection to process control to scientific research to hydrologic monitoring in surface waters (including lakes, rivers, and estuaries), ground waters (including aquifers and water tables), and waters in control structures (including pipes, channels, and tanks). Sensor drift is a debilitating and expensive problem affecting sensors and sensor drift caused by fouling is of particular concern. Fouling occurs when materials in the water(foulants) attach themselves to, and/or grow upon, a sensor to such an extent that the accuracy of the measurement deteriorates. Foulants include algae, bacterial slimes, fungi, oils, greases, grit, biological solids, or industrial by products such as paper fibers. Such fouling can render the sensor inoperable, or even worse, allow subtle errors to contaminate a data set and cause erroneous data analysis. Either way, the purpose of the sensors—accurate and timely data at a reasonable cost-is negated. Critical data is lost, and sensor operation and maintenance costs must increase to prevent additional data loss.
By way of example only, and not by limitation, many environmental, academic, and economic decisions require water-quality data. Some practitioners prefer to gather their water-quality data in situ, meaning that the pertinent water-quality sensors must be immersed in the water to be analyzed. Often, however, the water to be analyzed contains biological and mechanical, living and inert, contaminants that can accumulate and/or grow on the surface of the sensors, causing the sensors' calibrations to drift, and possibly rendering the data useless or subtly misleading. Practitioners often prefer to use multiple sensors in their investigations, so that a full set of data is gathered as quickly and inexpensively as possible. Instruments employing more than one sensor are called “multiprobes”. When multiprobes are deployed in waters containing foulants, the sensors will eventually become contaminated/fouled.
A variety of prior art attempts have been made to solve the problem of sensor fouling. When sensors are deployed without protection against fouling they can only be deployed in fouling conditions for short periods. This means that the frequency, and therefore the cost and safety risk, of retrieving, maintaining, and recalibrating the sensors is maximized. Solutions include mechanical devices such as shutters to shield the sensor from the fouling media as much as possible. Other devices include chemical means such as anti fouling paints, biocidal coatings, construction materials unfriendly to micro biota, and even electric fields to prevent the deposition and/or growth of biological foulants. Other sensors utilize other mechanical devices, such as compressed air or ultrasonic cleaners, to remove accumulated foulants. Problematically, these prior art mechanical devices are difficult to apply to multiple sensors of different sizes and shapes, raise the price and complexity of individual sensors, complicate calibration, maintenance, repair, and operation even in non-fouling situations, and often suffer from high power consumption and poor control.
So-called chemical solutions in use to date as a means to prevent the deposition and/or growth of biological foulants have little or no effect on non-biological foulants, including oils and sedtiments. For example only, prior art devices utilizing biocidal coatings are incapable of preventing foreign materials from accumulating in the vicinity of the sensor or doing anything about an inert film, such as oil, that prevents accurate operation of the sensor just as much as any living material. A similar problem exists with conventional optical sensors in that they can become contaminated by materials in the fluid, such as a algae in biologically active waters or sediment in industrial process waters. This problem is complicated by the fact that the optical sensor may be only slightly altered and cause the operator to accept errant or misleading data or draw erroneous conclusions.